Memory types and Installing

 By Lee Brannon

 Revised and rewritten - Posted 1/31/09

Last year I was working on an older computer and I added Three 128 Megabyte DIMM’s to it.  On reboot the computer would not boot past the BIOS.  One of the 128 DIMM's was the wrong type. That prompted me to put this information together to help people with Memory types.

There is no such thing as a memory expert. I am not a memory expert.  I went out and researched a lot of material and read a lot of material just like you are reading this.  I can also promise you that the next time I have amemory issue or question I will have to refer to my own notes.  We all just learn what we can. Here are some of the things I have learned in the course of time and generally out of necessity.

There are about as many different memory DIMMs, Simms, SODIMMs and RIMMs out there as there are people on the planet, or at least it seems that way. Putting the wrong memory into a machine is probably one of the easiest and most common mistakes.

Basically, when building a Frankenstein computer from parts there are several things you need to know before purchasing memory.

1) How many memory slots does the computer have?
2) What Mhz will your computer allow the chips to run in?
3) How many Pins do the chips need to have?
4) What is the maximum amount of Ram your computer/motherboard allow?
5) What type of Chip does the computer need.

Let’s attack this from a fictitious build. Your cousin has just given you his old Celeron 500Mhz IPAQ Compaq. (I could pick one of these Pyramid type Compaq’s up on Ebay for less than $20, so your cousin is a cheapskate. Oh well, it was free.) When you first get it booted you notice that it only has 64MB of RAM in it. Since the minimum for Windows XP is 128MB (I have seen it run on 64MB) you obviously need to do something.

Now since it’s a Brand name computer and you have enough information to go on. It should be a fairly easy process to find out all you need to know before buying chips for it. A few minutes on the web showed me that this system only has one memory slot. Yes, I could have opened the case and looked. This is generally bad news. It severely limits your options. Now for the worst news, the best it can handle is 256MB. (Yes, many computers have really low limits) The computer won’t recognize more than that. The good news is that we are giving this thing to Aunt Edna anyway and all she needs is something to browse the Internet to watch the exciting world of turtle races.

Don’t worry if this next paragraph loses you. . After all, what are the odds that your cousin is going to give you exactly the computer I described?

So, what do we need to know next? We need to know the type of chip or module, the number of pins, the Mhz speed, and whether or not it needs to be parity checking, non-parity, registered, ECC or EDU or etc. etc. Fortunately this particular computer uses a pretty standard 168 pin PC100 SDRAM DIMM that is non-parity and does not need any of the rest of the stuff.

Do you know what all that meant? If not, read my explanations as follows:

MHz - this is how fast the chip/module processess data. Basically you need a number that is the same or higher than the number your computer was designed for. If your Six year old machine was designed for 100 MHZ memory modules then you need 100 MHz or 133 MHz.

Number of Pins? Simply put your computer has memory slots or banks that the memory modules plug into. These slots have connectors to make the connections with the module. You have to use memory modules that have the same number of pins as the number of connections in the slot. Your average older desktop computer is probably going to have 168 connectors and the memory modules will need to be 168 pins. (Do some research on the net before buying anything.)

32 and 64 Bit Paths: Memory must talk to a controller or device using a certain number of data bits. Most use a 64 bit path. Some older memory modules only had 32 bit paths so memory for those machines had to be installed in pairs in the 64 bit machines. The basic SIMM is an example.

Chip types: SIMM, DIMM SDRAM etc

As I said before there are many different types of chip sets out there. We are only going to cover the most common memory modules found in the machines of the last decade or so.

You do not need to know what these letters stand for but it helps. One thing you should know is that SIMM was the most common Memory module type early on and many Techs use the term generically when talking about Memory modules of all types. Why, because it is easier to say than Memory Module.

Types of memory chips: DRAM, SRAM, SDRAM etc.

RAM (Random Access Memory) - Most memory is RAM. It means simply that any part of the memory can be accessed without having to read sequentially through the other parts. Also the generic term used by many when discussing any memory.

DRAM (Dynamic Random Access Memory ) - Must be refreshed often and thus makes it slower than SRAM. It also requires constant electrical current or it’s contents are lost.

SRAM (Static Random Access Memory) – Does not need to be refreshed and is much faster than DRAM but in the past it was always much more expensive and seldom used.

SDRAM ( Synchronous Dynamic Random Access Memory ) - This has pretty much been the standard memory module for the last 10 years. It synchronizes itself to the bus speed and runs much faster than DRAM.

Types of memory Modules: SIMM, DIMM SDRAM etc


A memory module is a small circuit board with memory chips connected to it. They generally have teeth or pins that connect it into a slot or bank on the motherboard of the computer.

SIMM (Single Inline Memory Module) – Generally this is module used in older Pentium computers with 30 or 72 Pins.

DIMM (Dual Inline Memory Module) – This is a single memory module with twice the bit path of a SIMM and can generally be used by itself. DIMM’s com in 72 pin, 144 pin and 168 pin modules.

SODIMM (Small Outline Dual Inline Memory Module) Basically this is a DIMM that is smaller in physical size and used mostly in laptop and notebook computers.

Again, there are many others, especially when you get to high end servers, but those I have listed are the most common.

So now that you know that your computer uses a certain number of pins and a certain type of chip you should be able to go get one right? Wrong. As if that was not enough to confuse you there are several variations of all of those Memory modules.

Now on to the things that will trip you up.

What is CL? CL stands for CAS Latency. It tells you how many clock cycles must pass before data is ready to be transferred. A CL 2 means that when the computer ask for the data in a memory location the clock cycle will pass twice before the data is ready. A CL 3 means that the clock cycle will pass three times. Some computers require a CL 2 or faster chip in which case a CL 3 will not work.

What is EDO? It stands for Extended Data Out. In simple terms with traditional DRAM memory there is a delay caused when the computer ask for information because the information must be returned before the computer can make another request. EDO modules allow the computer to start a second request while the first on is being processed.

What is Non-parity? Non parity memory is the most common. There is only one bit “spot” for each bit of data. No error correction or checking is done.

What is party? Parity is a form of error correction that uses an extra bit to check the contents of each 8 bits of data. So for every 8 bits of data, 9 bits are used on the module. The extra bit can be addressed directly.

What is ECC ? ECC is a form of parity checking that is often confused with parity. It has the extra bit like parity, but that extra bit cannot be accessed directly. ECC is popular and will sometimes work in non ECC machines, but the general rule of thumb is to only use ECC memory modules in ECC only machines.

What is Registered? Registered modules have an extra buffer chip on them that stores data for one clock cycle before passing it on. They have the benefit of allowing more memory access and generally have less electrical drain. Often used in Servers despite the speed delays. Some ECC memory is also registered memory, some ECC memory is not.

Note about “Buffered”: You can go out on the net and find dozens of postings that tell you that registered memory is buffered memory and the opposite, that they are not the same. As near as I can determine the term “buffered” is simply an older term used to describe registered.


So, now do you understand what to put in the computer your cousin gave you?
 

Common questions:

Why can't I just put as much memory into the machine as I can fit? Well, for one thing you will be wasting money. For example, I wanted to put 256MB SDRAMs into all three slots on the computer I was using to type this on, but when I checked i found that the machine was only designed to handle a maximum of 384MB of RAM. (Three 128MB SDRAMs .) Even if it would have run with the 256Mb modules I would have spent at least three times the money for nothing.

Why does my computer have 4 slots and two are one color and two are another?  This is an indication that your computer is using Dual Channel architecture.  Basically you can only add memory to the computer in "matched pairs.  Let's say you have two 1 GIG chip sets.  You need to add them both to the same color slots.

What if I put more memory in than the computer can handle? More than likely the computer will run, but in some cases it won’t boot.

I put the chips in and the Bios reports that the correct amount is there, but then the computer goes to a blank screen and never loads windows? This one is tricky. You could have the wrong type of modules installed or the memory could be bad. The Bios test of memory modules when you boot is generally not very extensive. Basically it checks to see if there are modules in the banks and what size the memory is. If you are sure you have purchased and installed the correct modules then get a memory testing program. I recommend MEMTEST33 (See my “Test that memory” section under Windows XP on this site.)

Can you mix Memory Speeds? As long as the memory modules meet all the other needs (ECC or non-ECC, Parity or Non-parity, CL’s etc.) then the general answer is yes. Keep in mind however that your faster modules will run at the speed of your slowest module.

If I have three different size chip sets and three slots does it matter what order they go in? Probably not, but I always stick with biggest to smallest. Start with Bank 0 and put the largest chip set or module in it. In theory this one will get the most use, so keeping everything in this first chip set should speed things up.